This invention relates to an electromagnetic valve, and particularly to an electromagnetic valve for use for discharge of molten metal from a container.
In British Patent Publication GB-A No. 777213 there is disclosed a method of controlling or preventing discharge of molten metal from a container through a discharge passage in the container below the level of the molten metal. The method comprises utilizing electromagnetic forces induced in the molten metal by an induction coil disposed around the container to move the molten metal away from the discharge passage in the container. When the coil is not energized the molten metal flows out of the container through the discharge passage under the action of gravity, but when the coil is energized the molten metal is moved away from the discharge passage, and there is no outflow.
When the magnetic field is applied to drive the metal away from the discharge passage, an air/metal interface is formed. As the denser molten metal is above the air, this free surface is inherently unstable. The surface tension and density of the molten metal, plus the magnitude and frequency of the applied magnetic field, determine the maximun extent of the surface for which it remains stable. Typically the maximum dimension of the free surface cannot exceed more than a few tens of millimeters, and this imposes a maximum size on the discharge passage to order to achieve the maximum flow rate required while retaining the ability to shut off the flow by applying the magnetic field.
In French Patent Publication FR-A No. 2316026 there is disclosed such a valve comprising a body providing a discharge passage through which, in use, molten metal will flow from a container under the action of gravity; an electrical induction coil located about the passage; and means to supply a high frequency electric current to the coil, whereby the coil provides an alternating magnetic field which induces electrical currents in molten metal in the passage, interaction between the filed and the currents providing a force which urges the molten metal away from the wall of the passage towards the axis thereof. An electromagnetic overpressure is thus created in the molten metal in the passage, which overpressure can be used to regulate the flow of the molten metal from the container.
In this document it is stated that the frequency f of the electric current supplied to the coil must be sufficiently high for the depth of penetration .delta. of the magnetic field into the moltem metal to satisfy the condition: EQU .delta.&lt;R (1)
where R is the radius of the molten metal stream in the passage before it is caused to contract by the application of the electromagnetic field.
The relationship between the frequency and skin depth is .delta.=.vertline.1/f.pi..mu..sigma. from which it follows that: ##EQU1## where .mu. is the magnetic permeability of the molten metal and .sigma. is the electrical conductivity of the molten metal.
Tests show that to achieve efficient flow control, the skin depth .delta. should be equal to or less than 1/3 of the radius R of the molten metal stream in the passage: ##EQU2##
To summarize, the current state of the art teaches that the frequency of the electric current should be sufficiently high for the skin depth to be small compared with the radius of the molten metal stream in the passage.
For the vast majority of molten metal discharge operations, the metal stream diameter lies between 13 and 20 mm. For ferrous alloys, for example, the frequencies to satisfy the equality expressed in (3) therefore lie in the range 80 to 30 kHz. For non-ferrous metals, such as aluminium for example, the frequency range is 15 to 6 kHz. The main interest in electromagnetic flow control valves is for the high melting point alloys, of which the ferrous alloys are the most important. For these alloys, field strengths as high as 1/3 Tesla might be needed to obtain the required degree of flow control. Currents of a few thousand amps will generally be needed to generate such field strengths. This combination of high current and high frequency poses a difficult electrical engineering problem. The induction coils used as small and have inductances of only a few microhenries, while matching transformers cannot be placed close to the molten metal stream. Thus, a low inductance bus-bar must generally be used to supply the electric current to the coil. A further problem, resulting from the high frequencies required, is that the power dissipated in the coil and the molten metal stream can become very large.